docs: def _find_longest_internal_edge(self, node):
          '''return the node that has the longest branch length between the given
          node and the root
          
          Parameters
          ----------
          node: dendropy.Tree
              a node from the tree
              
          Returns
          -------
          The node that has the largest length between the node and the root_node
          '''
          max_length = -1
          max_edge = None
          while node and not node.edge.rootedge:
              if node.edge.length > max_length:
                  max_edge = node.edge
                  max_length = max_edge.length
              node = node.parent_node
          return max_edge